1. The Field of the Invention
This invention relates generally to the field of thin film transistors (TFT), and more particularly, to a method of forming a number of thin-layer materials used in the fabrication of thin-film-transistors (TFTs) for devices such as a liquid crystal display (LCD).
2. Description of the Related Art
In the manufacture of thin film transistor (TFT) liquid-crystal-displays (LCDs), the active region of the display consists of an array of pixels, built on a glass substrate. The TFTs are made by various typical semiconductor processes on the glass substrate.
Before fabricating the TFTs, in general, the glass substrate needs to be coated with a base-coat (BC) layer to prevent outdiffusion of impurities, typically present in the glass material, to the TFT layers. The most typical base-coat material is silicon dioxide that is deposited by conventional CVD (chemical vapor deposition) processes.
Unfortunately, the silicon dioxide formed in accordance with such conventional methods often has an unacceptably high water content degrading the performance of the TFTs. Thus, to obtain good TFT characteristics and reliability, it is essential that the water be removed from the BC layer.
For this reason, conventionally, the BC layer deposition is usually followed by extra annealing (densification) processes before it comes in contact with the subsequently deposited TFT layers.
However, such extra annealing processes complicate the fabrication of the TFTs and become a source of contamination. This is because, while the glass substrate is annealed and carried between annealing furnaces and deposition chambers, the surface of the BC layer is exposed to an ambient and accordingly is contaminated. Further, the removal of the water from the base-coat layer is difficult with such conventional methods, especially when the substrate is exposed to an ambient.
In other approaches, the use of a dual BC layer has been suggested as a way to alleviate the need for post-deposition annealing processes. In this case, a SiNx/SiO2 stack is deposited. The SiNx layer demonstrates good barrier properties, whereas the SiO2 layer mainly improves the interface quality between the BC stack and the subsequently deposited TFT layers. This approach, however, requires deposition of two layers leading to more process steps and still cannot completely eliminate the —OH radicals trapped in the layers, as a result of the deposition chemistry.
Accordingly, there is a need for more efficient methods of forming thin film materials such as BC and active layers, while reducing the water or Si—OH content therein, with fewer or simplified process steps.